A quantum dot (Qdot) is a semiconductor crystal whose size is on the order of just a few nanometers. Qdots contain anywhere from 100 to 1,000 essentially free electrons and range from about 2 to 10 nanometers in size, or about 10 to 50 atoms, in diameter.
One of the optical features of excitonic Qdots noticeable to the unaided eye is coloration. While the material which makes up a quantum dot is significant, more significant in terms of coloration is the size. The larger the Qdot, the redder (the more towards the longer wavelength end of the electromagnetic spectrum) they fluorescence. The smaller the dot, the bluer (the more towards the short wavelength end) it is. The coloration is directly related to the energy levels of the Qdot. Quantitatively speaking, the bandgap energy that determines the energy (and hence color) of the fluoresced light is approximately inversely proportional to the square of the size of the Qdot. Larger Qdots have more energy levels which are more closely spaced. This allows the Qdot to absorb photons containing less energy, i.e. those closer to the red end of the spectrum.
Regarding fabrication, in semiconductors, Qdots are small regions of one material buried in another with a larger energy (band) gap. Qdots sometimes may occur spontaneously in quantum well structures due to monolayer fluctuations in the well's thickness. Self-assembled quantum dots nucleate spontaneously under certain conditions during molecular beam epitaxy (MBE) and metallorganic vapor phase epitaxy (MOVPE), when a material is grown on a substrate to which it is not lattice matched.
Being quasi-zero dimensional, quantum dots have a sharper density of states than higher-dimensional structures. As a result, they generally have superior transport and optical properties, and are currently being researched for use in diode lasers, amplifiers, and certain biological sensors.
More than a decade-long studies on colloidal luminescent Qdots have revealed that effective surface passivation is critical in making Qdots extremely bright and photostable. Continuous passivation coatings of sufficient thickness thus provide a reduction in the sensitivity of Qdots to various environmental conditions. As a result, Qdots are often passivated with a continuous coating to form core/shell structure comprising an encapulating shell having the Qdot core within.